The 2.7km long Zuckerberg II Sewer Tunnel in Stuttgart, Germany, successfully came on-line in September 2003. The 3.4m excavated diameter tunnel has been built to boost the capacity of the district’s only existing sewer line – the Zuckerberg I Tunnel, built some 85 years ago.

The project was commissioned by the client, the Urban Drainage Department of the Civil Engineering Authority of the state capital Stuttgart who awarded the US$21.8M contract to the Tunnelling Division of domestic contractor, Ed Züblin, in 2000.

The tunnel’s construction presented the contractor with a host of challenges. Not only was the geology far more complex than originally thought, requiring modifications to the machine mid-drive, but Züblin also utilised what is thought to be a ‘first’ for producing the tunnel lining.

Tunnel driving

The 2.6m i.d. Zuckerberg II Tunnel, in the city’s north, runs at a gradient of 0.04% between depths of 25m-80m from the dock in Bad Cannstatt in a northerly direction to Hofen (Figure 1) where the wastewater flows through a grit chamber into a central treatment plant. The alignment passes through a ridge high above the valley plain of the river Neckar, placing boring entirely within the Upper Muschelkalk limestone. Apart from finding a slight corrosion of individual joints, the preliminary investigations showed the rock quality to be good, with no karstification.

The project tender proposed either NATM or TBM as alternative construction methods.

For NATM to compete with a TBM in terms of excavation time, Züblin felt it would have had to excavate simultaneously from both tunnel ends. Subsequently the contractor put forward, and had accepted, the quicker and cheaper full-face mechanical tunnelling method, followed by the installation of a cast-in-place inner shell with an HDPE inner lining.

Work began at the Hofen Grit Chamber in January 2001 with the construction of a temporary bridge across the river Neckar, to provide a direct connection with the area’s road network. Underground excavation started later, in April, with the NATM construction of the 30m long TBM assembly tunnel. The contractor chose a 3.4m diameter Wirth TB II H open, full-face hard rock TBM to bore the tunnel, which, together with the trailer, measured some 125m in length. The cutterhead was equipped with 34 disc cutters (6 centre cutters, 25 face cutters and 3 calibre cutters) and a scraper and shovel system (muck scoops) for taking up the rock chips. Two cross-braces and eight grippers were used to align the TBM, ready for the tunnel drive.

Driving problems

Boring started from Hofen in mid May 2001. The TBM initially proved more than suitable for the alignment’s limestone. Little support was required through the rock, although anchors and shotcrete were required occasionally to support the crown and roof. The TBM’s best advance rate through the early stages reached 46.8m/day or 218.5m/week. Driving work was carried out continuously from Monday to Saturday, with Sundays reserved for maintenance.

Unfortunately, after 1,270m, the TBM encountered unforeseen karstification features coupled with fault zones, which occurred with little or no warning, which displayed the following three characteristics:

  • subsidence masses consisting of limestone boulders mixed with a varying loamy matrix

  • highly fractured limestone rock, with irregular, partly discordant bedding and a relatively low loam content

  • homogeneous loams and clays virtually free of stones

    The length of these discontinuities ranged from 10m -30m and by the end of boring, the machine had cut through 10 discontinuity zones, totalling over 200m.

    When boulders and fractured rock were present, the issue was mainly with the TBM cutterhead tearing large pieces of rock from the face – due to the low bonding strength of the rock. This sometimes led to considerable over excavation of the face and in some cases, large loose blocks of rock had to be broken up manually. The discontinuity zones were subsequently stabilised with shotcrete, although conditions in the crown became severe enough in two instances to require forepoling.

    Occasionally, the invert consisted of loamy-clayey material which was too soft to bear the weight of the TBM. At these points, to prevent the machine sinking into the ground, the floor profile was re-built manually and strengthened with shotcrete.

    The main problem facing the TBM drive through these discontinuity zones was the transfer of the gripper forces into the rock (required for machine advance). It was essential that the profile in the area of the grippers did not suffer any significant deformations. In addition to filling the overbreak with shotcrete and providing a reinforced shotcrete outer shell on the roof and sidewall, the entire tunnel cross-section, through the poor ground, was supported with bolted liner plates. These support measures proved successful, and relatively easy to install.

    Tunnel boring was aided by geological and geophysical ground investigations, to predict the rock conditions ahead of the TBM and develop concepts for tunnelling through any predicted discontinuities.

    The advance works involved 38m long core drilling from the face. Although this required extensive modifications to the TBM causing down-time, the information obtained was very valuable in dealing with one particularly critical discontinuity zone. The accompanying investigations also included vertical drilling from the surface at two points.

    Besides horizontal drilling, subsurface exploration also employed Tunnel Seismic Prediction (TSP) ahead of the machine. This was complimented above ground by hybrid seismic mapping – comprising reflection and refraction seismics – carried out over a 600m stretch along the tunnel axis.

    Despite these sophisticated investigations, it was not possible to predict and pinpoint all of the discontinuities. It became repeatedly necessary to react quickly to sudden unexpected changes in the rock conditions.

    The various fault zones, and subsequent modifications made to the TBM, held the tunnel drive up by four months. The TBM successfully broke through into Bad Cannstatt in April 2002.

    Tunnel lining works

    The permanent lining works began a month later in May 2002. Due to the relatively long tunnel and shallow gradient of 0.04%, it was critical to produce a system with both high durability and strong resistance to wastewater. The tender had proposed the installation of glass-fibre reinforced plastic pipes (GRP), with the annular space between the pipes and the rock filled with insulating material.

    As an alternative and cheaper proposal, Züblin submitted and had accepted an innovative concept incorporating a 250mm minimum thickness inner shell of un-reinforced cast-in-place concrete with a 4mm thick HDPE concrete protection liner (CPL). The concrete inner shell acts as support and the protective HDPE lining assures impermeability and durability.

    Works proceeded counter to the tunnel drive direction using the track laid during excavation for equipment transportation.

    The 250mm thick inner shell, with compression joints in the longitudinal direction, was created in a continuous cycle. The unreinforced concrete shell, with HDPE liner, was installed using a specially developed circular formwork system from Bystag, which was adjustable to the different required block lengths. These lengths were approximately 7.5m for the tunnel’s 200m radius curves, and approximately 15m for the 1000m radius curves and on the straight tunnel stretch.

    For the upper arch of the tunnel cross section (everything but the invert), the HDPE embedded concrete lining was prepared on site for an entire 7.5m or 15m long section, depending on the tunnel curve, and transported together with two precast elements to the formwork car in the tunnel.

    After formwork positioning, openings for the roof grouting holes, formwork supports and concreting connections were cut out from the inside the formwork. The concrete was then conveyed via hoses from the concrete pump in the tunnel to the formwork car and then into the concreting connections in the formwork. The cavity left in the roof after concreting was subsequently filled by grouting.

    For casting the invert lining, a 7.5m long, 100mm- 150mm thick precast concrete element was placed with the HDPE concrete protection liner embedded on its inner side.

    When concreting was complete, the radial block joints and the longitudinal joints, where the concrete protection liner of the arch connects to the precast element of the tunnel invert, were welded together using extrusion welding and checked for tightness.

    Concrete technology

    During the lining design phase it became apparent that the team would need to employ highly flowable concrete to fill the annular space. Factors considered included; the limited freedom of movement in the formwork car and the consequent need to restrict the number of concreting connections; the damping of the vibration energy of the external vibrators, due to the HDPE concrete protection liner placed on the formwork car; and the risk of deformation of the HDPE liner during the actual concrete placement.

    For this purpose Züblin developed a concrete with a slump of approximately 700mm (as compared with 550mm-600mm slump of the flowable concrete normally used in tunnel construction), as well as being self-compacting.

    For practical reasons, suitability tests allowed for fresh concrete temperatures of up to 35°C. This is due to the noticeable climate changes experienced in Central Europe that can lead to high fresh concrete temperatures during the hot summer months.

    As a result, several concretes were developed making it possible to provide a uniform fresh concrete quality despite the seasonal temperature variations.

    Such highly flowable concretes must be used simultaneously with a comprehensive quality assurance system. Therefore, the concrete slump of every single concrete mixer lorry was checked upon arrival on site and, if required, adjusted to the specified consistency in accordance with specially developed proportioning tables.

    In addition, the bulk density, void ratio and w/c ratio of the concrete were permanently monitored. The extent of checking exceeded the minimum laid down in DIN 1084 by some margin.

    On the basis of the experiences with the different types of concrete it was possible to draw the following conclusions.

    Easily compactable concrete has proved to be excellent for filling the annular space. The vibration energy transmitted via the face contact material and the HDPE concrete protection liner was adequate for compacting the concrete. Furthermore, the very high flowability of the concrete contributed to the complete filling of the tunnel blocks – confirmed by subsequent quality checks using georadar.

    In view of the increasing risk of deformation of the HDPE concrete protection liner when using lower slump mixtures, conventional flowing concrete with a slump of 550mm to 600mm would not have been suitable.

    In some blocks self-compacting concrete was also tested. However, this could not be used on the main stretch of the tunnel because of the sulphate content in the rock water. Self-compacting concrete in the form of ready-mix concrete is much more difficult to handle on site compared with easily compactable concrete. This type of concrete is very sensitive to any changes, however slight, in the different constituents.

    Moreover, the good results obtained with easily compactable concrete made the further use of self-compacting concrete unnecessary.


    The unexpected events that occurred during the construction of the Zuckerberg II Tunnel are a good example of the importance of getting the right balance between the predicted alignment geology and the choice of an appropriate TBM.

    Major differences in the expected geology can cause crippling rises in cost. The various discontinuities that were encountered during this project were dealt with using the support measures described, resulting in the TBM drive being completed safely, if prolonged by more than four months.

    Another special feature of this project was the installation of an inner shell consisting of un-reinforced cast-in-place concrete with an HDPE liner. This is thought to be the first time this lining construction method has ever been employed during the constructing a wastewater tunnel.

    The close cooperation between the client and the contractor enabled an economically advantageous implementation of the Zuckerberg II Tunnel project and fully achieved the desired result – a durable, high standard wastewater tunnel designed to maintain a reliable supply to the city of Stuttgart.

    Related Files
    Figure 3 – Schematic cross section of the lining design
    Figure 1 – Location of the Zuckerberg II Tunnel
    Figure 2 – Longitudinal section of the tunnel